Heretofore, polyether polyols, which are starting materials for polyurethane elastomers, adhesives, paints, sealants, and so on, have been produced by polymerization of alkylene oxides such as ethylene oxide or propylene oxide by use of initiators having active hydrogens. Double metal cyanide complex catalysts (hereinafter also referred to as DMC catalysts) are well known as a typical polymerization catalyst for the alkylene oxides. The DMC catalysts are catalysts containing an organic ligand and metal compounds, and are typified by compounds in which zinc hexacyanocobaltate (Zn3[Co(CN)6]2) is coordinated with an organic ligand, water and zinc chloride. Such DMC catalysts are produced, for example, by a method of mixing an excessive amount of an aqueous solution of zinc chloride with an aqueous solution of alkali metal hexacyanocobaltate, optionally in the presence of the organic ligand, to precipitate a solid, and then mixing the organic ligand or its aqueous solution with the solid to coordinate the organic ligand thereto.
It was recently reported that the life of the DMC catalyst became remarkably long when t-butyl alcohol was used as the organic ligand (Patent Document 1). Furthermore, Patent Document 2 describes that use of a highly active DMC catalyst with an organic ligand such as t-butyl alcohol decreases an amount of the catalyst used and eliminates a need for a process of removing the DMC catalyst after production of polyether polyols.
In the production processes of the DMC catalysts, an alkali metal halide as a by-product is also dissolved in a slurry containing the DMC catalyst, the organic ligand and water, obtained by reacting an aqueous solution of zinc chloride with an aqueous solution of an alkali metal hexacyanocobaltate, and then reacting the organic ligand or an aqueous solution thereof with the reaction product. A cake containing the DMC catalyst is separated from the slurry by such a method as filtration or centrifugal separation. However, unless the separation by filtration or centrifugal separation is enough, some of the alkali metal halide remains in the cake containing the DMC catalyst. It is known that it acts as a catalyst poison and causes deterioration of polymerization activity (Patent Document 3).
This remaining alkali metal halide can be removed by washing the cake containing the DMC catalyst with the organic ligand or the aqueous solution thereof. However, if the filterability of the DMC catalyst is poor, the washing efficiency will drop. Accordingly, there have been demands for DMC catalysts with good filterability. DMC catalysts coordinated with glyme as the organic ligand are obtained in the form of crystalline particles and it was thus industrially satisfactory in terms of the filterability. However, DMC catalysts coordinated with t-butyl alcohol as the organic ligand (hereinafter also referred to as t-butyl alcohol type DMC catalysts) have poor filterability and the slurry containing the DMC catalyst, water and t-butyl alcohol obtained through the production process is often obtained in the form of sticky paste, posing a problem that it is very difficult to separate the filter cake from the filtrate in a separation process of filtering the DMC catalytic component from the slurry.
Patent Document 4 suggests a method for producing DMC catalysts which can be readily filtered by controlling the order of addition of an aqueous solution of zinc chloride and an aqueous solution of an alkali metal cyanometalate, a reaction temperature and a stoichiometric ratio of Zn/Co in producing the DMC catalysts. However, as described in Patent Document 5, the above method is effective for the DMC catalysts with glyme as the organic ligand, but it was not effective for the t-butyl alcohol type DMC catalysts, because the resultant slurry containing the formed DMC catalyst became as gelatinous as difficult to be separated, with increase of the ratio of Zn/Co.
Furthermore, Patent Document 5 describes that in the production of the t-butyl alcohol type DMC catalysts, the Zn/Co ratio is decreased in preparation or excess zinc chloride is washed away in washing steps of the cake containing the DMC catalyst, thereby forming crystalline particles and providing the DMC catalyst with high activity and without the problem of difficulty in filterability. However, even though the separation by filtration became relatively easy, there was a problem that control of an amount of zinc chloride coordinated was difficult; for example, in a case where the amount of zinc chloride coordinated is too small, the crystallinity becomes too high, thereby losing the activity.
Furthermore, Patent Document 6 describes t-butyl alcohol type DMC catalysts and DMC catalyst-containing cakes with t-butyl alcohol and water, in which at least 90% of particles of the DMC catalyst have the particle size within the range of from 0.1 to 10 μm. In the invention, the DMC catalyst is not separated from the cake by filtration, but the DMC catalyst-containing cake is separated by centrifugal separation which is not suitable for industrialization.
On the other hand, Patent Document 7 describes a process for continuously and stably producing catalysts.
Patent Document 1: JP-A-04-145123
Patent Document 2: JP-A-2000-513389 (WO97/23544)
Patent Document 3: U.S. Pat. No. 4,472,560 (JP-A-58-185621)
Patent Document 4: U.S. Pat. No. 5,158,922 (JP-A-06-41292)
Patent Document 5: U.S. Pat. No. 5,627,122 (JP-A-09-31185)
Patent Document 6: U.S. Pat. No. 5,639,705 (JP-A-2000-513647)
Patent Document 7: JP-A-03-89946